1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive engines, and more particularly to an improved structure of a spark plug with a noble metal chip welded to at least one of a center electrode and a ground electrode which provides higher durability at a joint of the noble metal chip to the one of the center and ground electrodes, and a fabricating method thereof.
2. Background Art
There are known spark plugs with a noble metal chip welded to at least one of a center or a ground electrode. Japanese Patent First Publication No. 6-188062 teaches, as illustrated in FIGS. 27 and 28, irradiating a laser beam at given time intervals to a contact between a noble metal chip 2a and an electrode 2 rotating together to form a plurality of spot welds 200, as shown in FIGS. 29 and 30, which are made of materials of the noble metal chip 2a and the electrode 2 melted together. Each of the sport welds partially overlaps an adjacent one, thereby forming a complete annular alloy band extending over the whole of a circumference of the contact between the noble metal chip 2a and the electrode 2.
The above laser welding, however, encounters drawbacks in that much time is consumed in irradiating the laser beam to the noble metal chip 2a rotating together with the electrode 2 to form the spot welds 200 over an overall circumference of the contact between the noble metal chip 2a and the electrode 2, thus resulting in an increase in manufacturing cost of the spark plugs. Additionally, misalignment of the noble metal chip 2a with the electrode 2 will result in a variation in focus of the laser beam on a plurality of portions of the contact between the noble metal chip 2a and the electrode 2 to be spot-welded, thereby causing, as shown in FIG. 30, discrete interfaces 201 to be formed between the spot welds 200 which result in an increase in thermal stress acting on an interface between the noble metal chip 2a and each of the spot welds 200.
Further, in modern engines, a combustible atmosphere is elevated in temperature for increasing an output and reducing a fuel consumption and exhaust emissions. In this type of engine, a park plug is subjected to an intense heat, so that the temperature of center and ground electrodes is increased greatly. The electrodes, therefore, undergo a thermal stress and oxidation, which may cause noble metal chips to be removed from the center and ground electrodes.